Tin-plated steel plates have heretofore been used as a container material, etc., and in the commercial production thereof, the ferro-stann method, which is acid-bath plating technique, is extensively used.
The ferro-stann method uses a tin phenolsulfonate bath as the tin-plating bath. Although soluble tin electrodes were conventionally used as the anode, methods using insoluble electrodes, such as a platinum-plated titanium electrode, in place of the soluble electrodes, have recently been developed and come to be placed into practical industrial use.
However, this plating method using such insoluble anodes is still incomplete and should be improved further in some respects, although the method is very effective in eliminating the drawbacks accompanying the use of soluble electrodes. That is, there is a problem in that the consumed amount of phenolsulfonic acid (PSA), ethoxy-.alpha.-naphtholsulfonic acid (ENSA), etc., which are ingredients contained in the plating bath, is still considerably large, resulting in an insufficient reduction in the used amount thereof. In addition, there has been found to exist another problem, in that even when platinum-plated titanium electrodes are used, tin oxide sludges are formed in the plating bath, and this raises concerns that accumulation of such sludges in the bath or deposition thereof on the electrode surface may impede the plating operation and impair the quality of the tin-plated steel plates being produced.